class_docstr: A node in an expression tree.
  
  ExprNode always holds an operator. Leaves are strings.
docstr: Returns True if `expr` is a subtree.
docstr_tok: ["Returns","True","if","`","expr","`","is","a","subtree","."]
code: def is_in(self, expr):
      """Returns True if `expr` is a subtree."""
      if expr == self:
          return True
      is_in_left = is_in_expr(self.left, expr)
      is_in_right = is_in_expr(self.right, expr)
      return is_in_left or is_in_right
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from collections import namedtuple
  import random
  import six
  from six.moves import range
  import sympy
label_desc: Use image node_id 3 for calling the ExprNode obj's underlying member method code with example usage: obj.is_in(expr) and returns: unknown, True